Steel pickling method

ABSTRACT

A method and bath for cleaning and conditioning steel prior to tinplating in which the strip is pickled in a dilute aqueous solution of nitric and sulfuric acids. The pickled strip, when tinplated, has improved corrosion resistance.

United States Patent [1'91 Rauch, Jr. et al.

[ Aug. 14, 1973 STEEL PICKLING METHOD Inventors: Stewart E. Rauch, Jr.; Edward H.

Mayer, both of Bethlehem, Pa.

Assignee: Bethlehem Steel Corporation,

Bthlehem, Pa.

Filed: Oct. 29, 1971 Appl. No.: 194,065

Related US. Application Data Division of Ser. No. 45,929, June 12, 1970, Pat; No. 3,660,253.

u.s. Cl. 134/3, 134/41 Int. Cl 023; 1/08 Field of Search 134/3, 41; 252/142 References Cited UNITED STATES PATENTS 9/1956 Hahn 134/3 FOREIGN PATENTS OR APPLICATIONS 458,940 12/1936 Great Britain Primary Examiner-Morris o. Wolk AssistanrExaminer-D. G. Millman Attorney-Joseph J. OKeefe 4 Claims, No Drawings STEEL PICKLING METHOD This is a division of application Ser. No. 45,929, filed June 12, 1970, now US. Pat. No. 3,660,253.

BACKGROUND OF THE INVENTION This invention relates to a bath composition and method for conditioning or treating of the surface of steel prior to electrotinning, and more particularly to a method of pickling strip, which will provide, after plating, a material capable of being made into a superior sanitary food container. Steels used in the electrotinning process are in general characterized as low carbon steels. They have an iron content which is at least 95 percent of the total composition. A typical tin plate base material known in the art as Type L" steel has an analysis as follows:

C-0.12 max. Mn0.60 max. P-0.015 max.

S0.05 max. Si0.0l max. Ni0.04 max. Cu0.06 max.

the remainder being essentially iron.

In the production of electrotinplate, the steel strip base material, which has been cold rolled to gage, annealed, and usually given a temper roll, is cleaned of dirt and oil, and pickled in dilute mineral acid. After pickling, the strip is thoroughly rinsed in water and then introduced into an electrotinning bath. Pickling, preceding electrotinning, is usually performed in an aqueous solution of approximately 3 to 5 weight percent sulfuric acid.

Electrotinplate can be made from either an alkaline or an acid plating bath with the latter type gaining favor due to the greater speed at which acid tinplate can be produced. However, in the tinplate industry alkaline tinplate has generally been considered to have corrosion resistance superior to that of acid tinplate. This superiority is of particular importance in the shelf life of unlacquered tin cans containing the more corrosive types of acid food, such as fruit juices. Prior art acid tinplate used for such acid foods often fails after a relatively short shelf life.

This observed difference in corrosion characteristics between alkaline and acidelectrotinplate has led to the establishment of certain standards, which govern the use of electrotinplate for specific use.

The term Grade K is now used by certain can manufacturers to designate that tinplate grade which meets requirements of can shelf life for the more corrosive acid types of food and beverages, including citrus fruits and tomatoes.

To meet Grade K requirements, tinplate must pass several tests. Grade K" tinplate must have an iron solution value (ISV) of not more than 20. Iron solution value represents the dissolved iron in micrograms in a test solution of sulfuric acid containing ammonium-thiocyanate and hydrogen peroxide, after exposure of one side of a tinplate sample of specific dimensions to 50 milliliters of the test solution in a closed jar for 2 hours at 80 F. The ISV test is the subject of a discussion in Corrosion, Vol. 12, No. 9, September l956, pp. 23-30, the test being described in Appendix B The Iron Solution Test at p. 29.

Grade K tinplate must have, additionally, an alloy-tin couple" (ATC) value of not more than 0.12.

The alloy tin couple value represents the current flow in microamperes per square centimeter between a tin anode and the tin-iron alloy of a tinplate sample, after immersing a tinplate sample of specific dimensions, from which the free tin has been removed, in citrus juice for 20 hours. The ATC test is discussed in Corrosion," Vol. 17, No. 2, Feburary 1961, pp. 106-414, and the test is described in Appendix A Detailed ATC Procedure at pp. 113-114.

A further test of importance for Grade K tinplate is the pickle lag, i.e., the time between the immersion of a steel strip in a pickling solution and the time begins strip begins to pickle. This test is of particular importance in an acid plating line because of the increased speed involved in this type of line. The testing procedure for determining pickle lag entails immersing a detinned specimen in 6 N. hydrochloric acid at C. and measuring the time lapse before a steady rate of iron dissolution, or hydrogen evolution, is attained. The test procedure is discussed in Tinplate Testing (May 1960) by W. E. Home, Tin Research Institute, Middlesex, England. Preferably, Grade K tinplate should have a pickle lag of no more than 10 seconds. Heretofore tinplate made from steel which has been treated by a prior art pickling solution, i.e., generally sulfuric acid, has not consistently met Grade K requirements.

SUMMARY OF THE INVENTION It is a primary object of this invention to provide a treatment for cold rolled, annealed steel strip, which when subsequently electrotinned and flow brightened, will have a low iron solution value.

It is a further object to provide a treatment for cold rolled, annealed steel strip, which when subsequently electrotinned and flow brightened, will have improved ATC values.

It is another object of this invention to provide for the aforementioned strip a treatment wherein the pickle lag time is reduced.

It is another object of this invention to provide a treatment for cold rolled, annealed steel strip, which steel when tinplated will pass the aforementioned Grade K requirements.

It is another object of this invention to provide a pickling bath composition which will accomplish the above named objects.

The method of this invention comprises pickling a cold rolled annealed strip, from which dirt and oil have been removed, in a heated aqueous solution of from 0.5-l .5 weight percent nitric acid, calculated as nitrate (N0 ion, and from 25-20 weight percent sulfuric acid percentH S0 The strip is held in the pickling solution, for a predetermined time period, after which the pickling solution is thoroughly rinsed from the strip. The strip is then ready for plating.

The improved tinplate obtained by the use of the pickling bath composition and method of this invention is unexpected. Neither sulfuric nor nitric acid pickling baths alone will provide a steel base, which, when tinplated, will consistently pass Grade K" test requirements.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred method by which this invention can be performed is given in the following detailed description.

Cold rolled, annealed steel strip, having a gage of 0.0082 inch, and a carbon content of 0.08 weight percent, is cleaned cathodically in an aqueous solution of alkaline cleaner, as is well known to those skilled in the art. Thereafter, the strip is thoroughly rinsed with water and passed through rubber-coated squeegee rolls. The strip is then introduced into an aqueous pickling bath, at a bath temperature of approximately 150 F., comprising 10 grams per liter (1 weight percent) nitric acid and 50 grams per liter (5 percent) sulfuric acid. The strip remains in the pickling bath for approximately 5 seconds. It is then withdrawn, thoroughly rinsed with water, and excess water removed by passing the strip through rubber-coated squeegee rolls. The thus pickled and rinsed strip is then passed through an acid electrotinning line where the strip is coated with tin and subsequently flow brightened.

By use of the pickling method just described, followed by electrotinning and flow brightening, we have produced tinplate having an iron solution value not exceeding 20, and an alloy-tin couple value not exceeding 0.12.

The outstanding improvement in ISV and improvement in ATC value obtained with the nitric-sulfuric acid pickling bath and method, as against results obtained with conventional sulfuric acid pickling, for comparative samples, tinned from the same acid lines is shown from test results comparing the two pickling procedures.

For the comparative tests, box annealed 0.0082 inch gage blackplate having a carbon content of 0.08 percent was used.

Blackplate test pieces were made up in the form of cylinders. Each cylinder had a diameter of 6 inches and a height of 6 inches, the cylinder being subjected, to a height of 5 inches, to the test procedure given below.

The test procedure comprised fixing each cylindrical test piece in a vertical position on a rotating current conducting head, after which the test cylinder was cleaned electrolytically in a proprietary alkaline cleaning solution (Pennsalt 86-B), rinsed in water, pickled, rinsed, plated electrolytically with tin on both sides from an acid plating bath and rinsed. A number of thus treated cylinders were then cut up into panels having a dimension of 4 inches by 17.5 inches. The panels were flow brightened in an induction furnace, and from these panels, specimens were obtained upon which determinations were made for iron solution values.

The following table contains the results of tests run.

of 10 samples) HNO,, l0 gm./l. 175 11 Series VII (Average) H 50 100 grn./l.

of 10 samples HNO,, l0 gm./l. 175 6 Series VIII (Average) H,SO 200 gm./l.

of 10 samples HNO l0 gmJl. I 7

The preceding table shows the improved ISV properties of the combination of sulfuric and nitric acid pickling baths. Series I in which nitric acid was used, and Series II in which sulfuric acid was used, show ISV values well above the value of 20 established as the maximum allowable for Grade K tinplate. Series III and IV compared with Series V and VI show that an increase in temperature tends to lower the ISV value, other factors being the same. A comparison of Series III with Series V and Series IV with Series VI shows that an increase in percent I-INO tends to lower ISV values, other factors being the same. The last two series (VII and VIII) show that an increase in percent H also tends to lower the ISV value, other factors being the same.

All the ISV values represent an average of ten samples. These samples were, in each series, from five test cylinders which were then run in duplicate.

While the bath is operable with a sulfuric acid concentration up to 200 g./l. (20.0 percent), the preferred range for sulfuric is from 3 to 7.5 weight percent, as the faster pickling rate with higher concentrations of sulfuric acid may cause undesirable etching, with consequent loss of brightness of the strip surface.

In addition, the following two tests were made of steel samples in a production line test. In the first test, samples were taken from edges and centers of fourteen coils of tin plate. These samples were treated in a sulfuric acid heated bath. In the second test, sodium nitrate was added to the above bath to bring the nitrate concentration (Nflf) ion to 1 percent, the equivalent of 10 gm./1., and the bath temperature was raised to 180 F. Twelve coil samples were then pickled in this second bath, wherein all other operating conditions were the same as for the first test. The results of these tests are given in Table II below.

The above test shows the improvement in ISV, ATC, and pickle lag achieved by the bath and method of our invention over that of the prior art sulfuric acid bath.

In the practice of our invention the temperature of the pickling bath for best performance preferably should be from about F. to F. Temperatures above this range may be used, however, the temperature should be maintained below the point at which undesirable etching occurs and at which acid would be lost by excessive volatilization. The pickling time is also a variable, dependent on bath composition, bath temperature and pickling line speed, but generally is between about 2 to about 5 seconds.

It is also possible in the bath and method of this invention to substitute an alkali nitrate for nitric acid as a source of nitrate ions.

Other variations are also possible without departing from the scope of the novel concepts of this invention, such as the use of continuous annealed in place of box annealed plate. Also, this invention is suitable in the production of alkaline tinplate as well as the acid tinplate of the examples.

We claim:

1. The method for the pickling of steel base material prior to electrotinplating comprising immersing said steel in an aqueous solution of about 0.5 to 1.5 weight percent nitric acid calculated as nitrate (N0 ion and about 2.5 to 20.0 weight percent sulfuric acid calculated as 100 percent H 80 at a solution temperature above about l50 F.

2. The method as claimed in claim 1 further comprising maintaining the immersing step for a period of from about 3 to 5 seconds at a temperature of about 150 F. to 180 F.

3. The method as claimed in claim 1 wherein the weight percent sulfuric acid is from about 3.0 to 7.5.

4. The method for the production of cold rolled annealed strip which comprises the steps of:

a. pickling cold rolled annealed steel strip in an aqueous solution of about 0.5 to 1.5 weight per cent nitric acid calculated as nitrate (N0 ion and about 3.0 to 7.5 weight per cent sulfuric acid calculated as percent H 80 at a solution temperature between about F. to about F. for a period of time to give an iron solution value not exceeding 20 micrograms. 

2. The method as claimed in claim 1 further comprising maintaining the immersing step for a period of from about 3 to 5 seconds at a temperature of about 150* F. to 180* F.
 3. The method as claimed in claim 1 wherein the weight percent sulfuric acid is from about 3.0 to 7.5.
 4. The method for the production of cold rolled annealed strip which comprises the steps of: a. pickling cold rolled annealed steel strip in an aqueous solution of about 0.5 to 1.5 weight per cent nitric acid calculated as nitrate (N03 ) ion and about 3.0 to 7.5 weight per cent sulfuric acid calculated as 100 percent H2S04 at a solution temperature between about 150* F. to about 180* F. for a period of time to give an iron solution value not exceeding 20 micrograms. 